phosphor-hwmon: Fix unit escaping

We aren't correctly escaping all of the specical characters based on the
systemd spec for unit name escaping. systemd provides a utility to do
this reliably, so use that instead of a custom implementation.

(From meta-phosphor rev: e1acbce6ac818e411cee8483c89ea6f7b0829162)

Change-Id: I239f396f92e6a6b94edcfe48084c41dcc292e1b7
Signed-off-by: William A. Kennington III <wak@google.com>
Signed-off-by: Brad Bishop <bradleyb@fuzziesquirrel.com>